Green’s Dictionary of Slang

say adv.

yes.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 208/1: If it is, I say, ‘Say.’ That is, – Yes. If not, I say ‘Nanti’.
1870		Hotten Sl. Dict.
1890		Northampton Mercury 12 Apr. 12/4: ‘Shif,’ fish; ‘torrac,’ carrot; ‘yad,’ day; ‘kool,’ look; ‘sey,’ yes; ‘on’, no; ‘pac,’ cap; ‘reeb,’ beer, ‘yob,’ boy; are all perfect specimens [i.e. of backslang].
1953		J. Franklyn Cockney 298: Had the manager replied Say (Yes) instead of On (No) the customer would have gone away happy.
2010		W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 59: Backslang is a language form that writes or pronounces words backwards, for example, ecaf (face), riah (hair), say (yes), yob/yobbo (boy).